Ships built specifically to transport liquid cargoes in bulk have existed since the early 1900's. The majority of such vessels are used to carry crude oil or refined petroleum products. Until the 1960's their size was dictated by the limitations of the Suez Canal. After the Suez Crisis and the Arab-Israeli War the size of these ships increased dramatically to the point where bulk oil transports are now the largest ships afloat. Overall lengths in the order of a quarter of a mile and beams in excess of 200 feet have earned them the title "Supertanker". Fully loaded, many Ultra Large Crude Carriers (ULCC) have deadweight tonnages greater than the combined displacement of six "Nimitz" or "Eisenhower" class, nuclear powered aircraft carriers. Their extreme weight and length makes them ponderous to manuever. Once under way, it takes many miles to bring them to a halt. Drawing 80 to 90 feet, they are more prone to grounding than other vessels. The consequences of large volume oil spills are too well known. Their incredible cost in terms of lost cargo, clean-up expense and permanent environmental damage indicates there is a need for an effective solution. Considering the number of ULCC's plying the oceans, the potential for even larger spills is an ever present danger.
The risk of oil spills began with the launching of the first tanker. It is only the magnitude of recent spills that has caused world governments to seriously consider precautionary measures aimed at preventing their recurrence. Today's dominant global thinking is to mandate that all tankers have double hulls by some future date. However, double hulls are not a universally embraced solution. Building them into new ships or retrofitting existing tankers with inner hulls will be extremely expensive. Also, the cargo carrying capacity of double hulled ships is less than that of single skinned ships of the same displacement. The increased cost of double hulls, coupled with a reduced payload, will add to the cost of transporting oil. A more valid concern is that, while double hulls give a sense of absolute protection, they in fact do not. Considering the enormous momentum of a supertanker, it is highly probable that with a heavy grounding, both bottoms can be torn open. Similarly, it is likely that a severe collision would easily slice through both hulls.
In spite of the general belief that double hulls are the best insurance against future tanker spills, there is still a clear need for a means of preventing such spills that:
1. will be less expensive, both to retrofit in existing tankers or to build into new construction;
2. permits utilization of a tanker's cargo carrying capacity to its full potential; and
3. offers as good if not better protection against a wider variety of spill producing accidents.
The infrequency of major oil spills since the advent of the supertankers, has given credence to the opinion that they pose little risk and sustained the myth that clean-up facilities were adequate. That lack of spills until recently had reinforced the position of the oil industry that no special safety precautions were needed. In the past many anti-spill measures have been suggested, but none have succeeded in achieving any degree of acceptance or adoption by the shipping industry.
One such solution is found in U.S. Pat. No. 4,241,683, to Conway, titled "Liquid Cargo Tank Construction", which discloses a liquid cargo vessel having a hull comprising a bottom and sides, a top deck, and a number of transverse bulkheads disposed within the hull to form a plurality of water-tight cargo compartments between the top deck and the hull bottom. The specific improvement taught in this patent is the provision of a water-tight horizontal deck disposed within the hull between the top deck and the bottom deck above the waterline of the tank vessel and at a distance above the hull bottom which is less than or equal to H(S.sub.w S.sub.c), where H represents the vertical height of the waterline of the vessel above the hull bottom, S.sub.w represents the specific gravity of water, and S.sub.c represents the specific gravity of the liquid cargo. The horizontal bulkhead constituting the improvement forms separate upper and lower water-tight liquid-receiving tanks within compartments above and below the waterline of the vessel.
In a vessel constructed as disclosed in the Conway patent, with the lower tank full, the pressure on the bottom due to the weight of the cargo will just equal the upward sea pressure against the hull. Should the tank's bottom be punctured, then with the outward and inward pressures in balance and because the oil will float on the heavier sea water, little or no cargo will escape. Had the tank been less than full, sea water coming in would force the cargo upward and no cargo would be lost. In that case a void will be left between the cargo surface and the top of the tank. If however that same vessel is holed at some point in its side above its bottom and below the waterline, the outward pressure of the cargo will be slightly greater than the inward sea pressure. The liquid contained within the holed compartment is inclined to flow out. This creates a below atmospheric pressure at the underside of the top of the damaged lower tank. If the cargo contains volatiles such as are frequently found in crude oil, or should the lower tank be vented to atmosphere, then the liquid's surface can move down from the tank's top. Cargo will flow out until a hydrostatic balance at the hole is reached. After that, cargo will continue to ooze out the upper part of the hole at the same rate seawater flows in the lower half of the hole and settles to the bottom of that side tank. This continues until all of the liquid cargo below the top of the hole has been displaced upwardly and out. The lost cargo will equal the air or vapor space above the remaining cargo plus all the oil that had been below the upper extremity of the opening. Should a lower side tank, as disclosed in the Conway patent, have a hole that extends close to the waterline, then virtually all the cargo in that damaged lower side tank will be lost.